A large number of reasons exist for lighting a large underwater environment including security, safety and illumination of work surfaces. Applications include oil drilling platforms, lighting around submarines and ships and for storage pools. In all applications it is desirable to use a high efficiency, long lifetime light source which can provide continuous lighting with minimal maintenance. Nowhere is the need for a low maintenance lighting system more pronounced than in nuclear spent fuel storage pools and in nuclear reactor vessels, in which water is used to slow the reaction rate, where service of the lighting system results in radiation exposure for maintenance personnel.
Typically, these pools require a large number of lights for effective illumination. Traditionally this lighting has been accomplished using 1000 W, 120 V incandescent spotlights or floodlights. These bulbs have lifetime ratings of 2,000 to 4,000 hours, and provide total light output of 17,000 lumens. At a lifetime of 4,000 hours, a particular light fixture will require 2.19 bulb changes per year, with maintenance personnel being exposed to radiation at each bulb change. A typical fuel storage pool uses 20 incandescent light fixtures. Thus, maintenance personnel are subjected to short periods of radiation quite frequently for single bulb changes or to extended periods of exposure for "en mass" changes, if the bulbs are replaced at all.
In the reactor cavity of a nuclear reactor, water is normally contained only in the immediate area of the reactor itself, i.e., the reactor vessel. However, when the reactor is shut down to change the fuel, it is necessary to fill the entire reactor cavity to control the reaction rate of the replacement fuel as it is loaded. The reactor cavity is flooded only about four to five days out of a year, but it is necessary to make sure that lighting in the cavity is capable of safe and reliable operation in underwater or high humidity environments when such operation becomes necessary.
When maintenance is being performed on the reactor itself, and when the fuel is being changed, maintenance personnel do not have a lot of time to concern themselves with routine maintenance operations such as changing burned out light bulbs. In isolated areas where radiation can become quite high, access is available only for limited periods, and it takes several days to bleed off the radiation. When access is finally available, workers are concerned with maintenance of pumps and other equipment, and with the fuel change. Every minute of radiation exposure is critical, and excess radiation exposure can result in fines for the reactor operator and loss of manpower if a worker's cumulative radiation exposure exceeds a predetermined level. As a result, many light bulbs remain burned out, so that much of the structure is poorly illuminated. Even in areas where water is not introduced, a reliable, long lasting light source is replacements for the currently-used old-style incandescent bulbs.
A number of underwater lights are the subjects of patents, however, for various reasons, these lights are not suitable for use in nuclear environments, either as fixed lights or as drop lights. The submersible light assemblies of Olsson et al. (U.S. Pat. No. 4,683,523, issued Jul. 28, 1987 and U.S. Pat. No. 4,996,635 issued Feb. 26, 1991) have funnel-shaped housings with flared front portions designed for fixed attachment to submersible vehicles. The light sources are quartz-halogen lamps which require heat sinks, and the lamps themselves are fully isolated from water. The housings are relatively large and cumbersome and not adjustable in direction once attached. The light produced is generally projected in a narrow beam forward from the lens. Such a construction would not be suitable for the broad illumination needed in a nuclear pool or for the maneuverability required for a drop light.
The underwater light of Poppenheimer (U.S. Pat. No. 4,574,337, issued Mar. 4, 1986) has a housing that is much larger than the small quartz-halogen lamp housed therein. The lamp is fully isolated from the water by an inner casing which is cooled by water that enters the outer housing. The light is projected forward in a generally narrow beam, resulting in the same limitations for use in nuclear applications as the lights of Olsson et al.
The high-intensity light source described by Mula (U.S. Pat. No. 5,016,151, issued May 14, 1991) has a watertight housing with a second subhousing to isolate the lamp from the water. The flared shape of the housing places limitations on the maneuverability of such a device as a drop light.
Finally, and most importantly, none of the above-described lights make provisions for rapid changeout of burned out or damaged bulbs. The reliance on closed housing construction requires any bulb changes to be made out of the water, which is one of the main problems that must be overcome in a hazardous environment such as in nuclear facility pools. Such changes are time-consuming and require multiple radiation exposures to effect a bulb replacement. If the entire lighting assembly were to be replaced to avoid multiple exposures, such changes would be very expensive due to the complex construction of the assemblies. Any facility which required a large number of such light systems would find them to be prohibitively expensive.
High pressure sodium (HPS) lighting has been used extensively for street and parking area illumination, lighting in factories and for security lighting. The primary advantages of HPS lights are 1) high efficiency and 2) very long lifetime. Compared to a 1000 W incandescent bulb, an HPS bulb has a lifetime rating of 24,000 hours and provides a total light output of 140,000 lumens. Simple math emphasizes the advantages of HPS lights: one HPS bulb provides the light of eight incandescent bulbs for six to twelve times longer.